A mechanism for reducing a drive wheel torque according to racing of the drive wheels so as to prevent loss of acceleration performance is known as a drive torque controller or traction control system.
It is also known that the frictional coefficient .mu. of a road surface influences the slip factor of the drive wheels. In this connection, Tokkai Hei 4-55156 published by the Japanese Patent Office in 1992, discloses a device which estimates the frictional coefficient .mu. from the vehicle acceleration.
In this device, the estimation accuracy of the frictional coefficient .mu. is improved by using an acceleration in a predetermined short time and an acceleration in a predetermined long time according to the accelerating state of the vehicle. Also, the slip factor of the drive wheels is limited to a permitted range by, for example, decreasing the engine output according to the frictional coefficient .mu. of the road surface.
In this drive torque controller, however, a problem arises if the drive wheels race when there is a large resistance to vehicle motion or on a road surface with high resistance, such as for example a rising slope with a high frictional coefficient .mu.. This is because, when drive torque is reduced on a rising slope, the engine rotation cannot increase due to the large resistance to vehicle motion, and sufficient acceleration cannot be obtained. Also, some time is required for the drive torque to return its original level. As a result, some time is needed to reach a predetermined acceleration, and in addition to the loss of acceleration performance, the driver experiences discomfort.